Process and apparatus to remove closely spaced chips on a multi-chip module

ABSTRACT

A process and apparatus for removing flip chips with C4 joints mounted on a multi-chip module by applying a tensile force to one or more removal member bonded to the back of one or more flip chips during heating of the module to a temperature sufficient to cause the C4 joints to become molten. The tensile force can either be a compressed spring, or a bimetallic member which is flat at room temperature and becomes curved when heated to such temperature, or a memory alloy whose original shape is curved and which is bent flat at room temperature but returns to its original curved shape when heated to such temperature. An adhesive is used to bond the removal member to the chip to be removed and is a low temperature, fast curing adhesive with high temperature tolerance after curing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the repair of multi-chipmodules and, more particularly, to a process and apparatus for removingone or more chips from a multi-chip module in which the chips are soclosely mounted to each other on the module the chips are described asbeing brickwalled.

2. Description of the Prior Art

To increase the number of flip chips being C4 joined to a multi-chipmodule and thus increase the density without increasing the physicalsize of the module, the chip are being placed so close together that thesides of the chips are nearly touching each other. The removal of one ormore chips is required due a defective chip or C4 joint, discoveredduring testing, or to upgrade the module function by replacing one ormore of the mounted chips with one or more higher performing chips. Onepresent method and apparatus for removing chips from a module employgripping clips which are used in conjunction with a bi-metallic disc.One of the clips physically contacts one side of a chip to be removedand the other clip contacts the opposite side of the chip. When themodule is heated to a temperature to cause the C4 joints to becomemolten, a bimetallic disc provides an upward tensile force on thegripping clips to remove the gripped chip from the module. Examples ofchip removal by gripping the opposite sides of a chip is found in U.S.Pat. Nos. 4,485,958, 5,553,766 and 5,779,133, and IBM TDB Vol 18, No 9,February 1976, pp 2850.

However, because of the physical size of the gripping clips, a minimumspacing of 2 mm is required between chips in order to remove a chip withsuch gripping clips. This minimum spacing requirement wastes space onthe top surface of the module and prevents closely spaced mounting ofchips previously described as brickwalling.

Other processes and apparatuses for removing flip chips from a modulewithout gripping the opposite sides of a chip are hot vacuum asdescribed in U.S. Pat. No. 5,605,277 and hot gas as disclosed in U.S.Pat. No. 5,598,965. Neither of these processes and apparatus aresuitable for brickwall mounted chips because of a large spacing isrequired between chips so that the chip to be removed can be isolatedfrom the other chips.

For removing non-flip chips or chips without C4 joints in which the backof the chips are mounted to the module, a process and apparatus isdescribed in U.S. Pat. No. 5,154,793 which permits brickwall mountingbut, because this process and apparatus is used for removing back bondednon-flip chips, the temperature required is not sufficient high to meltC4 joints for the removal of flip chips in the temperature range ofabout 340 to about 375° C.

Consequently, there is a need for a flip chip removal process andapparatus which is able to remove chips from brickwall mounted chipswithout damaging the removed chips or chips adjacent to the removedchips. Therefore, it is an object of the present invention to provide alow cost, highly reliable process and apparatus which does not requiregrippers to remove the flip chips and which permit minimal spacingbetween chips to achieve brickwalling. A further object of the presentinvention is to provide a process and apparatus which pulls the flipchip from the back surface of the chip thereby not requiring any spacingbetween the chips. A final object of the present invention is provide aprocess and apparatus in which the conventional furnace for C4 joiningcan be used for removing selected flip chips while simultaneouslyjoining additional chips on other locations of the multi-chip module.

SUMMARY OF THE INVENTION

To achieve the foregoing objects, the present invention comprises theprocess of bonding a removal member, of a dimension smaller than thearea of the chip, to the exposed back of the chip with a low temperaturecurable thermal adhesive which, when cured is able to withstand the hightemperatures capable of melting the C4 joints. A tensile force isapplied to the removal member which is greater than the surface tensionof molten C4 joints of the chip or chips to be removed. The multi-chipmodule is then heated in an ambient controlled furnace to a temperatureto melt the C4 joints. The tensile force necessary will vary dependingupon the number of C4 joints. The larger the number of C4 joints, thegreater the tensile force required to overcome the surface tension ofthe C4 joints. Due the tensile force applied, either before or duringthe heating of the module, to the removal member on the backs of thechip or chips to be removed, the chip or chips will be lifted from themodule when the C4 joints become molten. After cooling, the lifted chipor chips are removed from the module while the C4 joints of theremaining chips are solidified to the pads on the module.

Another aspect of the present invention is an apparatus for removingbrickwalled chips comprising a removing assembly including a removalmember, bonding means for attaching the removal member to the back of achip, and tension means to apply a minimal tensile force to the removalmember and the back of the chip to be removed either prior to heatingthe module to a temperature to melt the C4 joints or, alternatively,during such heating.

A further aspect of the present invention is the process of removing achip from a multi-chip module comprising the steps of bonding one end ofa removal member to the back of the chip to be removed, followed byapplying a tension force to the other end of the removal member, beforeor during heating of the module to a temperature to cause the C4 jointsto melt, which is greater than the surface tension of melted C4 joints;and then heating the module to that melting temperature to separate thechip to be removed from the module.

BRIEF DESCRIPTION OF THE DRAWINGS

The above summary of the invention and other objects and features of thepresent invention will become apparent from the following description ofpreferred embodiments given in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a plan view of a multi-chip module with flip chips mounted onthe module in brickwall fashion with one identified for removal.

FIG. 2 is a cross sectional view of the multi-chip module of FIG. 1positioned with the preferred flip chip removal assembly of the presentinvention with a tension spring and on a fixture used for carrying themodule and removal assembly through the furnace to melt or reflow the C4joints.

FIG. 3 is a cross sectional view of the multi-chip module with thepreferred removal assembly after passing through the furnace and coolingof the C4 joints and showing the chip to be removed lifted off of themodule.

FIG. 4 is a cross sectional view of the multi-chip module of FIG. 1positioned with a pair of alternative flip chip removal assemblies ofthe present invention with bi-metallic discs or memory alloy discs andon a fixture used for the carrying the module and a pair of removalassemblies through the furnace to melt or reflow the C4 joints.

FIG. 5 is a cross sectional view of the multi-chip module with the pairof alterative removal assemblies after passing through the furnaceshowing the chips to be removed lifted off of the module.

DETAILED DESCRIPTION

A flip chip is one in which is the chip face, with the contacts of thecircuit, is mounted face down. The contacts are formed of a lead/tinalloy which, when melted, will reflow and maintain its location on thechip and are commonly know as Controlled Collapse Chip Connection (C4)joints. When attaching flip chips to a module, the chips are positionedon pads on the module and the module carrying the chips are passedthrough a furnace at a temperature sufficient to melt the lead/tin alloyof each C4 joint and reflow it to the respective pads on the module.Upon cooling the chips will be physically and electrically connected tothe module. Because the flip chips with C4 joints permit areaconnection, or contacts across the face area of the chip, to the module,the chips can be spaced so closely together on the module that theyresemble a brickwall or less than 0.4 mm spacing between chips.

With this general background, the present invention will now bedescribed in conjunction with the drawings. Turning to FIG. 1, the topof a multi-chip module 10 is shown with closely spaced or brickwalledflip chips 11 across the upper surface of the module. The back 12 of thechips are shown because the chips are flipped or mounted face down andthe C4 joints are not shown. Chip 11 a has been identified as one to beremoved. In FIG. 2, the preferred embodiment of the flip chip removalassembly 13 of the present invention is shown with a housing or tensionmount 14 with one or more access holes 15. A removal member 16, smallerin dimension than the area of the chip, extends through one of the holesand is in contact with the back 12 of the chip to be removed 11 a. Atension spring 17 is positioned around the member 16 with one end 18 incontact with the top of the housing or tension mount 14 and the otherend 19 in contact with an adjustable tension element 20 for compressingthe spring 17. The tension mount 14, compressed spring 17 and tensionelement 20 comprise tension means for applying a tensile force to theremoval member 16. During removal of the chip 11 a, the multi-chipmodule is mounted on a fixture 21 for carrying the module through aheating means or furnace (not shown) to reflow C4 joints 22 of the chips11. The tension mount 14 rests on the outer edges of the fixture 21.

Preferably, the removal member 16 comprises two pieces; one isrelatively flat block or disc 23 and the other is a threaded stud 24which is threaded into the block or disc 23 and on which is threaded thetension element 20 or herein, a nut. The block or disc 23 is smaller indimension than the area of the back 12 of the chip. In the presentinstant, the block or disc 23 is 1-2 mm smaller in each dimension of thechip.

In accordance with one aspect of the present invention, the block ordisc 23 is bonded to back 12 of the chip 11 a to be removed with anbonding means or a low temperature curable adhesive 25. Herein, the curetemperature is in the range of about 100 to about 160° C. and, so as tonot impact throughput in manufacturing, the cure time should be as shortas possible and normally should not exceed 15 minutes. Because theadhesive must withstand the temperature of the reflow furnace after ithas cured, the adhesive must retain bonding between the chip and theblock or disc 23 at temperatures as high as about 375° C. because thereflow furnace operates in a temperature range of about 340 to about375° C. to reflow the C4 joints of the chips 11. The adhesive 25 can bea commercially available epoxy adhesive which cures in 15 minutes orless at temperature in the range of 100 to 160° C. and which will retainit adhesion, after curing, at temperatures as high as about 375° C.

In operation, the tension on the spring 16 is determined empirically bythreading the element 20 down to compress the spring and then passingthe module 10 on the fixture 21 through the furnace a sufficient numberof times to identify the compression point on the spring and theposition of the element 20 on the threaded stud 24. This compressionpoint is when the tension of the spring is adequate to overcome thesurface tension of C4 joints 22 of the chip a to be removed. Thissurface tension varies depending upon the number of C4 joints on thechip. As shown in FIG. 3, the module 10 on the reflow fixture 21 hasbeen passed through the furnace after the compression point has beendetermined and the chip a has been removed from the module 10 while theother chips 11 remain fixed to the module.

After the fixture 21 has been removed from the furnace, the housing ortension mount 14 is separated from the module 10 on the fixture takingwith it the removed chip a. Using an organic solvent for the adhesive,such as heated xylene or parachloroethylene at a temperature in therange of about 80 to about 100° C., the adhesive 23 is dissolvedsufficiently to detach the chip a from the block or disc 23. The removalapparatus 13 is now ready to remove a chip from another module. Inaddition, the C4 joints of the removed chip, if operable, can bereworked and the chip reused. Although FIGS. 2 and 3 and theaccompanying description only shows and narrates the removal of onechip, it will be understood that a number of chips can be removed at onetime with the removal assembly 13. It would require additional holes 15in the housing or tension mount 14 aligned with the chips to be removedand additional members 16 adhesively attached to the chips to beremoved, springs 17 and tension elements 20, With this setup, aplurality of chips can be removed during one pass through the furnace.It should be understood that, although the multi-chip module 10 depictedin FIG. 1 shows all of chips centerlines lined up, the housing ortension mount 14 could be modified to so that any or all chips could beremoved even though the centerlines do not line up. Further, it shouldbe understood although not shown in the drawings that a chip or chipscan be added to the same multi-chip module during the removal of one ormore chips during one pass through the furnace.

An alternative embodiment of the present invention is show in FIGS. 4and 5 in which the spring 17 of removal assembly 13 (FIGS. 2 and 3) isreplaced with a layered bimetallic element 30 in which the layers havedifferent thermal expansion coefficients. In this embodiment, two flipchips are to be removed from a module as shown in FIGS. 4 and 5. Asshown in FIG. 4, removal assemblies 31 comprises a housing or tensionmount 32 containing one or more holes 33 a and 33 b in its top portionand aligned with one or more chips 11 a and 11 b to be removed. Removalmembers 34 a and 34 b extend through the holes aligned with the chips tobe removed and their lower ends 35 a and 35 b are adhesively bonded tochips and their upper ends 36 a and 36 b are rotatable attached totension elements 37 a and 37 b. Positioned between the tension elements37 a and 37 b and the housing or tension mount 32 are the layeredbimetallic elements 30 a and 30 b in the form of a disc. The discs 30 aand 30 b are relatively flat at room temperature, as shown in FIG. 4,but become curved shapes during and immediately after leaving the reflowfurnace (not shown as shown in FIG. 5). If necessary, washers 38 a and38 b can be used to extend the diameter of the tension elements 37 a and37 b. Depending on which side the layered bimetallic elements 30 a and30 b are positioned, the curvature of the element, when heated, eitherwill be concave, as shown by element 30 a in FIG. 5 or convex as shownby element 30 b in FIG. 5. The particular layered bimetallic element 30selected is based on the known curved shape the element 30 takes on atspecific temperatures. The selected element 30 is one whose temperaturecoincides with the melting temperature of the C4 joints 22 of the chips11 and is in the range of about 340 to about 375° C.

Alternatively, a shaped memory alloy of nickel-titanium alloy can beused in place of the bimetallic element 30. The memory alloy can be bentfrom its original shape into a different shape and, upon heating, itwill return to its original shape. In replacing the bimetallic element30 with the memory alloy, the original shape of the memory alloy iscurved and is bent into a flat configuration as shown in FIG. 4. Whenthe removal assembly passes through the reflow furnace, the memoryalloys, in place of the bimetallic elements 30 a and 30 b, will returnto their original curved shapes and a cause a tensile force on each ofthe removal members to separate the chip 11 a and 11 b from the modulein the same manner as shown in FIG. 5. The particular memory alloy isselected based upon the temperature at which the memory alloy returns toits original curved shape and is a temperature is in the range of about340 to about 375° C. or one coinciding with the melting of the C4 joints22. Again, depending on which side the flat memory alloy is positioned,the curvature of the heated memory alloy will be either concave orconvex as shown in FIG. 5.

In operation, the general process of the chip removal of the presentinvention comprises the steps of: 1) bonding a removal member to a chipto be removed on a multi-chip module positioned on a reflow fixture; 2)placing a tension mount having holes aligned with the chips on themodule on the reflow fixture; 3) extending the removal member throughthe hole in the tension mount aligned with the chip to be removed; 4)positioning a tension member, capable of applying a tensile forcegreater than the surface tension of the melted C4 joints of the chip,over the removal member and in contact with top of the tension mount;15) attaching an adjustable tension element to the free end of theremoval member and in contact with the tension member; and 5) passingthe reflow fixture carrying the module and removal apparatus through afurnace to melt the C4 joints of the chips on the module and to causethe tension force of the tension member on the removal member toseparate the chip to be removed from the module. A more detailed aspectof the process of the present invention is where the tensile force isthe result of either a compressed spring or the heating of a flatbi-metallic element or a flat memory alloy

Although this invention has been described relative to particularembodiments, it will be apparent to those skilled in the art thatvarious changes and modification may be made without departing from thespirit and scope of the invention as defined in the following claims.

Having thus described the invention, what is claimed is:
 1. Apparatusfor removing at least one flip chip of a plurality of closely spacedflip chips having C4 joints on their faces connected to pads on amulti-chip module comprising: a selective flip chip removal assemblyincluding a chip removal member and a tension mount having a top andfour sides surrounding said multi-chip module and having at least onehole in its top aligned with a chip to be removed, said removal membercomprising a block of a dimension smaller than an area of the back ofthe chip and a threaded stud fastened to the block at one end and to anadjustable tension element at the other end and extending through thehole in the top of a tension mount; bonding means for adhesivelyattaching said block to the back of the chip to be removed; tensionmeans disposed around said stud and between the top of the tension mountand the tension element for applying a tensile force to said removingmember greater than the surface tension of C4 joints while in a moltenstate; and heating means for heating the C4 joints to a temperature toplace the C4 joints in a molten state, whereby the tension means causesthe removing member to lift and separate the chip to be removed from themulti-chip module.
 2. The apparatus of claim 1 wherein the multi-chipmodule is mounted on a reflow furnace fixture and the tension mount is abox-like structure with its four sides positioned at and on the outeredges of said fixture.
 3. Apparatus for removing at least one flip chipof a plurality of closely spaced flip chips having backs and C4 jointson their faces connected to pads on a multi-chip module having a topcontaining pads and a periphery free of pads comprising: a flip chipremoval assembly including a removal mount disposed on the top andaround the periphery of said module and a chip removal member slidabledisposed in said removal mount and having an adjustable tension element;bonding means for attaching said removal member to the back of the chipto be removed, tension means disposed between said removal mount andsaid tension element for applying a tensile force to said removal membergreater than the surface tension of the C4 joints while in a moltenstate; and heating means for heating the C4 joints to a temperature toplace the C4 joints in a molten state, whereby the tension means causesthe removal member to lift and separate the chip to be removed from themulti-chip module.
 4. The apparatus of claim 3 wherein said heatingmeans is separate from the chip removal member.
 5. The apparatus ofclaim 3 wherein the multi-chip module contains at least one chip with C4joints not connected to pads on the module and said chip removalassemble is associated with a connected chip on the module to beremoved, whereby the unconnected chip is connected to the pads on themodule and said connected chip is removed after the heating means placesthe C4 joints of the unconnected and connected chips in a molten state.6. The apparatus of claim 3 wherein the tension means is a compressedspring acting on the removal member and the chip to be removed and isdecompressed when the surface tension of the C4 joints is overcomeduring heating to coact with the removal member to lift said chip off ofthe multi-chip module.
 7. The apparatus of claim 3 wherein the tensionmeans is a flat layered bimetallic element acting on the removal memberand the chip to be removed and is deformed into a curved shape duringheating to coact with the removal member to lift said chip off of themulti-chip module when the extent of the curved shape exceeds thesurface tension of the C4 joints.
 8. The apparatus of claim 3 whereinthe tension means is a flattened memory alloy element, whose memoryshape is curved, acting on the removing member and the chip to beremoved and returns to its curved memory shape during heating to coactwith the removing member to lift said chip off of the multi-chip modulewhen the extent of the curved shape exceeds the surface tension of theC4 joints.
 9. The apparatus of claim 3 wherein the bonding means forattaching the removing member to the chip to be removed is a lowtemperature, fast curing adhesive with high temperature tolerance aftercuring.
 10. The apparatus of claim 9 wherein the adhesive cures in atemperatures range of about 100 to about 160° C. in less than 15 minutesand can tolerate temperature as high as about 375° C.
 11. Apparatus forremoving at least one flip chip of a plurality of closely spaced flipchips having backs and C4 joints on their faces connected to pads on amulti-chip module comprising: a flip chip removal assembly including atension mount having a top with at least one hole aligned with a chip tobe removed, an adjustable tension element and a chip removal member,said removal member comprising a block of a dimension smaller than thearea of the back of the chip and a threaded stud fastened to the blockat one end and to the tension element at the other end and extendingthrough the hole in the top of the tension mount; bonding means forattaching the block of said removal member to the back of the chip to beremoved; tension means for applying a tensile force by said tensionelement to said removal member greater than the surface tension of theC4 joints while in a molten state; and heating means for heating the C4joints to a temperature to place the C4 joints in a molten state,whereby the tension means causes the removal member to lift and separatethe chip to be removed from the multi-chip module.
 12. The apparatus ofclaim 11 wherein said tension mount surrounds the multi-chip module andis positioned on the outer edges of a heating fixture along with themodule, the tension mount serving as a base for said tension means, andthe fixture carrying the module and removal assembly thus disposedaround the stud and between the top of the tension mount and the tensionelement.